An ophthalmologic surgical microscope of the kind referred to above is known from German patent publication DE 35 39 009 A1. There, an ophthalmologic surgical microscope is described which is equipped with an ancillary module which includes an ophthalmoscopic magnifier and a system for beam transposition and image reversion. The surgical microscope has a microscope main objective system having an intercept distance which can be adjusted. Accordingly, it is possible with the surgical microscope to sharply adjust the image of an intermediate image, which is generated by the ophthalmoscopic magnifier, or the image of an object region without it being necessary to move the surgical microscope or that additional optical elements are needed which have to be switched into the viewing beam path when the ancillary module is not in the viewing beam path.
Surgery on the human eye, for example, cataract surgery, is, as a rule, carried out utilizing a surgical microscope. The human eye is a spatially expanded organ which is accessible to a surgeon only from the side of the cornea. During the course of surgery, there is therefore the need for a surgeon to view sharply different planes in the eye.
The depth of field ST of a surgical microscope is dependent upon an adjusted magnification β and can be estimated as follows:
      ST    ≈                            0.5          ⁢          λ                          NA          2                    +                        0.34          ⁢                                          ⁢          mm                          β          ⁢                                          ⁢          NA                      ,wherein NA is the numerical aperture of the viewing beam path of the main objective of the surgical microscope and λ is the wavelength of the light with which an area of surgery is illuminated. For conventional magnifications of 5 to 30 times, there thereby results a depth of field which is less than the spatial expanse of the eye from the cornea to the ocular fundus. This condition has the consequence for the surgeon that the surgical microscope during the course of a surgical procedure must be refocused in correspondence to the plane in the eye at which surgery is just then taking place.
The physiological structures in an eye are substantially transparent. This makes it difficult for a surgeon to sharply adjust the surgical microscope onto different planes in the eye whereat surgery is taking place.
U.S. Pat. No. 6,212,006 describes a surgical microscope designed for eye surgery. The surgical microscope has a base body which is held on a carrier arm of a surgical microscope stand above the head of a patient to be operated upon. The surgical microscope has a microscope main objective via which a surgeon can view the patient eye to be operated upon via a stereoscopic viewing tube. The surgical microscope is equipped with an ophthalmoscopic magnifier which can be pivoted into or out of the viewing beam path. Because of the refractive power of cornea and lens in a human eye, an ophthalmoscopic magnifier arranged close in front of the eye must be used in order to be able to examine the ocular fundus with the surgical microscope. If, on the other hand, structures in the area of the cornea and lens of an eye are to be magnified with the surgical microscope, then the ophthalmoscopic magnifier is removed from the viewing beam path. In lieu of an ophthalmoscopic magnifier, so-called contact lenses are also used in eye surgery which are optical elements which are placed on the patient eye to be examined and which have a positive refractive power which cancels the refractive power of the cornea.
Autofocus systems in surgical microscopes are known. A surgical microscope having an autofocusing unit is described in German patent publication 201 11 006 U1. The autofocusing unit includes an image sensor on which the object region, which is examined by means of the surgical microscope, is imaged via a beam splitter prism having an autofocus beam path. An evaluation unit is assigned to the image sensor by means of which a contrast of an area of the image, which is detected by the image sensor, can be specified. The evaluation unit is connected via a control line to a drive for a displaceable lens in the microscope main objective system. The displaceable lens is always adjusted because of the signal from the evaluation unit so that the area of the image, which is detected by the image sensor, always has a maximum contrast.
United States patent application publication US 2004/0090667 A 1 discloses a surgical microscope wherein a focusing state is determined on an image sensor from the offset of a scanning light beam which is scattered in the object region. In this way, it is possible even with low contrast of an object region to detect the focusing state of the surgical microscope.
U.S. Pat. Nos. 5,867,308 and 6,005,710 disclose a surgical microscope having an autofocusing system wherein the autofocusing system can be activated by means of an eye control.
The fault-free function of such an autofocus system has as a condition precedent that either structures are present in the object region which cause a contrast or that the scanning light beam in the object region is so scattered that an occurring scatter light image can be captured by an image sensor. However, if an organ, which is transparent over wide regions such as the human eye, is examined, the functioning of an autofocus system is not guaranteed in each case because, not in all levels of the eye are there structures which have a high contrast or scatter a scanning light beam.